Backside-illuminated photodetector

ABSTRACT

The present invention provides a back illuminated photodetector having a sufficiently small package as well as being capable of suppressing the scattering of to-be-detected light. A back illuminated photodiode  1  comprises an N-type semiconductor substrate  10 , a P + -type doped semiconductor region  11 , a recessed portion  12 , and a coating layer  13 . In the surface layer on the upper surface S 1  side of the N-type semiconductor substrate  10  is formed the P + -type doped semiconductor region  11 . In the rear surface S 2  of the N-type semiconductor substrate  10  and in an area opposite the P + -type doped semiconductor region  11  is formed the recessed portion  12  that functions as an incident part for to-be-detected light. Also, on the rear surface S 2  is provided the coating layer  13  for transmitting to-be-detected light that is made incident into the recessed portion  12 . The coating layer  13  is here arranged in such a manner that the portion provided on the recessed portion  12  is sunk lower than the portion provided on the outer edge portion  14  of the recessed portion  12.

TECHNICAL FIELD

The present invention relates to a back illuminated photodetector.

BACKGROUND ART

In such a conventional back illuminated photodiode 100 as shown in FIG.24, in the superficial surface layer of an N-type silicon substrate 101are formed a P⁺-type highly-doped semiconductor region 102 and anN⁺-type highly-doped semiconductor region 103. The P⁺-type highly-dopedsemiconductor region 102 and the N⁺-type highly-doped semiconductorregion 103 are connected, respectively, with an anode electrode 104 anda cathode electrode 105. On the electrodes 104 and 105 are formed bumpelectrodes 106 made from solder. Also, the N-type silicon substrate 101is thinned in the portion corresponding to the P⁺-type highly-dopedsemiconductor region 102 from the rear surface side thereof. The thinnedportion functions as an incident part for to-be-detected light.

As shown in FIG. 24, the back illuminated photodiode 100 is packed intoa ceramic package 107 by flip-chip bonding. That is, the bump electrodes106 of the back illuminated photodiode 100 are connected to solder pads109 provided on a bottom wiring 108 of the ceramic package 107. Thebottom wiring 108 is connected to output terminal pins 110 through wirebonding. Also, on the surface of the ceramic package 107 is seam-weldeda window frame 111 using brazing material 112. In the window frame 111is formed an opening at the position corresponding to the thinnedportion of the back illuminated photodiode 100, and in the opening isprovided a transmissive window member 113 such as kovar glass fortransmitting to-be-detected light.

Patent Document 1: Japanese Published Unexamined Patent Application No.H09-219421

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in such an arrangement of using a ceramic package in a backilluminated photodiode as above, there is a problem in that the packagebecomes larger.

Meanwhile, in Patent Document 1 is disclosed a CSP (Chip Size Package)technique for semiconductor electronic components. This technique isadapted to seal the both surfaces of a wafer with semiconductorelectronic components built therein using organic material such asresin, and then to form an opening in the organic material provided onone surface side of the wafer by photolithography to form electrodestherein.

However, trying to apply such a CSP technique to a back illuminatedphotodiode to reduce the package size leads to the following problem.That is, the back illuminated photodiode is thinned at the portion thatfunctions as an incident part for to-be-detected light, which reducesthe mechanical strength thereof. Therefore, not a pyramid collet but aflat collet is used when assembling the back illuminated photodiode. Forexample, when heating and pressurizing a bump electrode, etc., providedon the surface side of a photodiode, heat and pressure is added from aheater block while the back illuminated photodiode sticks thereto usinga flat collet to employ the rear surface thereof as a sticking surface.

In the case of using a flat collet for a back illuminated photodiodewith the rear surface being sealed with resin, the resin may be damageddue to contact with the collet. If the resin in the thinned portion(i.e. incident part for to-be-detected light) of the back illuminatedphotodiode may thus be damaged, there is a problem in that the damagescatters to-be-detected light. Then, the scattering of to-be-detectedlight also leads to a reduction in the sensitivity of the backilluminated photodiode.

The present invention has been made to solve the above-describedproblems, and an object thereof is to provide a back illuminatedphotodetector having a sufficiently small package as well as beingcapable of suppressing the scattering of to-be-detected light.

MEANS FOR SOLVING THE PROBLEMS

In order to solve the above-described problem, the present invention isdirected to a back illuminated photodetector comprising: a firstconductive type semiconductor substrate; a second conductive type dopedsemiconductor region provided in the first superficial surface layer ofthe semiconductor substrate; a recessed portion for incidence ofto-be-detected light formed in the second surface of the semiconductorsubstrate and in an area opposite the doped semiconductor region; and acoating layer made of resin for transmitting the to-be-detected light,the coating layer being provided on the second surface, and the coatinglayer being arranged in such a manner that the portion provided on therecessed portion in the second surface is sunk lower than the portionprovided on the outer edge portion of the recessed portion.

In the back illuminated photodetector, since there is provided thecoating layer, the mechanical strength of the back illuminatedphotodetector can be increased. The increase in mechanical strengthallows for dicing at a wafer level, whereby it is possible to obtain achip-sized back illuminated photodetector. Accordingly, it is possibleto achieve a back illuminated photodetector having a sufficiently smallpackage. The coating layer is also made of resin for transmittingto-be-detected light not only to increase the mechanical strength of theback illuminated photodetector, but also to function as a transmissivewindow member for to-be-detected light.

Further, the coating layer is arranged in such a manner that the portionprovided on the recessed portion is sunk lower than the portion providedon the outer edge portion of the recessed portion. Therefore, even if aflat collet may be used in an assembling operation, the surface of thecoating layer provided on the recessed portion is not brought intocontact with the flat collet. Thus, the incident part for to-be-detectedlight within the surface of the coating layer cannot be damaged, wherebyit is possible to suppress the scattering of to-be-detected light.

The back illuminated photodetector according to the present inventionpreferably comprises a supporting film provided on the first surface ofthe semiconductor substrate to support the semiconductor substrate. Inthis case, the mechanical strength of the back illuminated photodetectorcan be increased.

Further, the back illuminated photodetector preferably comprises afilling electrode penetrating through the supporting film and connectedelectrically to the doped layer at one end thereof. In this case, it ispossible to take a detected signal easily outside the back illuminatedphotodetector.

It is preferable that a highly-doped semiconductor region withimpurities of the first conductive type added thereto at a highconcentration is exposed across the entire side surface of thesemiconductor substrate. In this case, even if a side surface of thesemiconductor substrate may be damaged through dicing, etc., thehighly-doped semiconductor region can trap unnecessary carriers that aregenerated in the vicinity of the side surface of the semiconductorsubstrate, and therefore can suppress dark current and/or noise.

It is preferred that a highly-doped semiconductor layer with impuritiesof the first conductive type added thereto at a high concentration isprovided in the bottom portion of the recessed portion within the secondsuperficial surface layer of the semiconductor substrate. Thehighly-doped semiconductor layer functions as an accumulation layer.This can make carriers generated upon incidence of to-be-detected lighteasy to move toward the first surface of the semiconductor substrate,resulting in an increase in the sensitivity of the back illuminatedphotodetector.

It is preferable that a highly-doped semiconductor layer with impuritiesof the first conductive type added thereto at a high concentration isprovided in the second superficial surface layer in the outer edgeportion of the semiconductor substrate. In this case, even if there maybe crystal defects in the vicinity of the second superficial surface inthe outer edge portion, the highly-doped semiconductor layer cansuppress dark current and/or noise due to the crystal defects.

EFFECTS OF THE INVENTION

In accordance with the present invention, it is possible to achieve aback illuminated photodetector having a sufficiently small package aswell as being capable of suppressing the scattering of to-be-detectedlight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a first embodiment of a backilluminated photodetector according to the present invention.

FIG. 2 is a view illustrating the effect of the back illuminatedphotodiode 1 shown in FIG. 1.

FIG. 3 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 4 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 5 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 6 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 7 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 8 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 9 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 10 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 11 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 12 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 13 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 14 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 15 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 16 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 17 is a step chart showing a method for manufacturing the backilluminated photodiode 1 shown in FIG. 1.

FIG. 18 is a cross-sectional view showing a second embodiment of a backilluminated photodetector according to the present invention.

FIG. 19 is a view illustrating an exemplary method for forming theN⁺-type highly-doped semiconductor region 28 shown in FIG. 18.

FIG. 20 is a view illustrating an exemplary method for forming theN⁺-type highly-doped semiconductor region 28 shown in FIG. 18.

FIG. 21 is a view illustrating an exemplary method for forming theN⁺-type highly-doped semiconductor region 28 shown in FIG. 18.

FIG. 22 is a plan view showing a third embodiment of a back illuminatedphotodetector according to the present invention.

FIG. 23 is a cross-sectional view of the back illuminated photodiodearray 3 shown in FIG. 22 along the line XII-XII.

FIG. 24 is a cross-sectional view of a conventional back illuminatedphotodiode.

DESCRIPTION OF SYMBOLS

1 and 2: Back illuminated photodiodes, 3: Back illuminated photodiodearray, 10 and 50: N-type semiconductor substrates, 11 and 51: P⁺-typedoped semiconductor regions, 12 and 52: Recessed portions, 13 and 53:Coating layers, 14 and 54: Outer edge portions, 20: Semiconductorsubstrate, 21 and 61: N⁺-type highly-doped semiconductor layers, 22; 28and 62: N⁺-type highly-doped semiconductor regions, 23; 24; 63 and 64:Insulating films, 25 and 65: Anode electrodes, 26 and 66: Cathodeelectrodes, 31 and 71: Passivation films, 32 and 72: Supporting films,33 a; 33 b; 73 a and 73 b: Filling electrodes, 34 a; 34 b; 74 a and 74b: UBMs, 35 a; 35 b; 75 a and 75 b: Bumps, S1: Upper surface, S2: Rearsurface, S3: Bottom surface of recessed portion, S4: Side surface of thesemiconductor substrate 20

BEST MODES FOR CARRYING OUT THE INVENTION

Preferred embodiments of a back illuminated photodetector according tothe present invention will hereinafter be described in detail withreference to the accompanying drawings. Additionally, in thedescriptions of the drawings, identical components are designated by thesame reference numerals to omit overlapped description. Also, thedimensional ratios in the drawings do not necessarily correspond tothose in the descriptions.

FIG. 1 is a cross-sectional view showing a first embodiment of a backilluminated photodetector according to the present invention. The backilluminated photodiode 1 is adapted to receive to-be-detected light fromthe rear surface thereof, to generate carriers upon incidence of theto-be-detected light, and then to output the generated carriers as adetected signal from the upper surface thereof. The back illuminatedphotodiode 1 comprises an N-type semiconductor substrate 10, a P⁺-typedoped semiconductor region 11, a recessed portion 12, and a coatinglayer 13. As the N-type semiconductor substrate 10, for example, asilicon substrate with N-type impurities such as phosphorous addedthereto can be used. The impurity concentration of the N-typesemiconductor substrate 10 is 10¹² to 10¹⁵/cm³, for example. Also, thethickness t1 of the N-type semiconductor substrate 10 is 200 μm to 500μm, for example.

In the surface layer on the upper surface (first surface) S1 side of theN-type semiconductor substrate 10 is partially formed the P⁺-type dopedsemiconductor region 11. The P⁺-type doped semiconductor region 11 isprovided with P-type impurities such as boron to form a PN junction withthe N-type semiconductor substrate 10. The impurity concentration of theP⁺-type doped semiconductor region 11 is 10¹⁵ to 10²⁰/cm³, for example.Also, the depth of the P⁺-type doped semiconductor region 11 is 0.1 μmto 20 μm, for example.

In the rear surface (second surface) S2 of the N-type semiconductorsubstrate 10 and in an area opposite the P⁺-type doped semiconductorregion 11 is formed the recessed portion 12. The recessed portion 12functions as an incident part for to-be-detected light. The recessedportion 12 has a shape that narrows gradually from the rear surface S2to the upper surface S1. More specifically, the recessed portion 12 mayhave, for example, a square pyramid shape or a tapered shape thatnarrows gradually from the rear surface S2 to the upper surface S1. Thedepth of the recessed portion 12 is 2 μm to 400 μm, for example. Also,due to the thus formed recessed portion 12, the area between the bottomsurface S3 of the recessed portion and the P⁺-type doped semiconductorregion 11 within the N-type semiconductor substrate 10 is made thinnerthan the other areas so that carriers generated upon incidence ofto-be-detected light via the rear surface S2 can easily reach near theP⁺-type doped semiconductor region 11 provided in the surface layer onthe upper surface S1 side. In addition, the thickness of the thinnedarea is 10 μm to 200 μm, for example.

On the rear surface S2 of the N-type semiconductor substrate 10 isprovided the coating layer 13. The coating layer 13 is made of resintransparent to to-be-detected light, that is, having a sufficienttransmissivity for the wavelength of to-be-detected light. As such resinepoxy-based, silicon-based, acryl-based or polyimide-based one, orcomposite material thereof can be cited. The coating layer 13 functionsas a protective layer for protecting the rear surface S2 as well as atransmissive window member for transmitting to-be-detected light that ismade incident into the recessed portion 12. Also, the coating layer 13is arranged in such a manner that the portion provided on the recessedportion 12 is sunk lower than the portion provided on the outer edgeportion 14 of the recessed portion 12. That is, the surface of thecoating layer 13 provided in the portion where the recessed portion 12is formed gets into the N-type semiconductor substrate 10 side deeperthan the surface of the coating layer 13 provided in the outer edgeportion 14 of the recessed portion 12. Herein, the outer edge portion 14indicates the portion laterally surrounding the recessed portion 12within the N-type semiconductor substrate 10. The thickness of thecoating layer 13 on the outer edge portion 14 is 5 μm to 500 μm, forexample, and preferably 250 μm.

The back illuminated photodiode 1 also comprises an N⁺-type highly-dopedsemiconductor layer 21, an N⁺-type highly-doped semiconductor region 22,insulating films 23 and 24, an anode electrode 25, and a cathodeelectrode 26. The N⁺-type highly-doped semiconductor layer 21 is formedin the entire surface layer on the rear surface S2 side of the N-typesemiconductor substrate 10. The N⁺-type highly-doped semiconductor layer21 is provided with N-type impurities at a concentration higher than inthe N-type semiconductor substrate 10. The impurity concentration of theN⁺-type highly-doped semiconductor layer 21 is 10¹⁵ to 10²⁰/cm³, forexample. Also, the depth of the N⁺-type highly-doped semiconductor layer21 is 0.1 μm to 20 μm, for example.

The N⁺-type highly-doped semiconductor region 22 is formed in thesurface layer on the upper surface S1 side of the N-type semiconductorsubstrate 10 at a predetermined distance from the P⁺-type dopedsemiconductor region 11. The N⁺-type highly-doped semiconductor region22 is also provided with N-type impurities at a high concentration, asis the case with the N⁺-type highly-doped semiconductor layer 21, to bea contact layer for the cathode electrode 26 to be described hereinafteras well as to have a function of suppressing surface leakage current inthe upper surface S1. The impurity concentration of the N⁺-typehighly-doped semiconductor region 22 is 10¹⁵ to 10²⁰/cm³, for example.Also, the depth of the N⁺-type highly-doped semiconductor region 22 is0.1 μm to 30 μm, for example.

The insulating films 23 and 24 are formed, respectively, on the uppersurface S1 and the rear surface S2 of the N-type semiconductor substrate10. The insulating films 23 and 24 are made of SiO₂, for example. Thethickness of the insulating film 23 is 0.1 μm to 2 μm, for example.Meanwhile, the thickness of the insulating film 24 is 0.05 μm to 1 μm,for example. Also, in the insulating film 23 are formed openings(contact holes) 23 a and 23 b, one opening 23 a being provided withinthe range of the P⁺-type doped semiconductor region 11, while the otheropening 23 b being provided within the range of the N⁺-type highly-dopedsemiconductor region 22.

On the insulating film 23 and in the areas including the openings 23 aand 23 b are formed, respectively, the anode electrode 25 and thecathode electrode 26. The thickness of the electrodes 25 and 26 is 1 μm,for example. The electrodes 25 and 26 are provided in such a manner asto fill the respective openings 23 a and 23 b. Thus, the anode electrode25 is connected directly to the P⁺-type doped semiconductor region 11through the opening 23 a, while the cathode electrode 26 is connecteddirectly to the N⁺-type highly-doped semiconductor region 22 through theopening 23 b. As the anode and cathode electrodes 25 and 26, forexample, A1, can be used.

The back illuminated photodiode 1 further comprises a passivation film31, a supporting film 32, filling electrodes 33 a and 33 b, UBMs (UnderBump Metals) 34 a and 34 b, and bumps 35 a and 35 b. The passivationfilm 31 is provided on the upper surface S1 of the N-type semiconductorsubstrate 10 in such a manner as to cover the insulating film 23, anodeelectrode 25, and cathode electrode 26. Also, in the portions providedon the anode electrode 25 and the cathode electrode 26 within thepassivation film 31 are formed through holes 31 a to be filled with thefilling electrodes 33 a and 33 b to be described hereinafter. Thepassivation film 31 is made of SiN, for example, to protect the uppersurface S1 of the N-type semiconductor substrate 10. The passivationfilm 31 can be formed by, for example, a plasma-CVD method. Also, thethickness of the passivation film 31 is 1 μm, for example.

On the passivation film 31 is formed the supporting film 32. Thesupporting film 32 is adapted to support the N-type semiconductorsubstrate 10. Also, in the portions corresponding to the through holes31 a in the passivation film 31 within the supporting film 32 are formedthrough holes 32 a to be filled with the filling electrodes 33 a and 33b that also fill the through holes 31 a. As a material of the supportingfilm 32, for example, resin or SiO₂, etc., that can be formed by, forexample, a plasma-CVD method can be used. Also, the thickness of thesupporting film 32 is 2 μm to 100 μm, for example, and preferably about50 μm.

The filling electrodes 33 a and 33 b fill the through holes 31 a and 32a, and are brought into contact, respectively, with the anode electrode25 and the cathode electrode 26 at one end thereof to be connectedelectrically to the P⁺-type doped semiconductor region 11 and theN⁺-type highly-doped semiconductor region 22. Also, the other end of thefilling electrodes 33 a and 33 b is exposed at the surface of thesupporting film 32. That is, the filling electrodes 33 a and 33 bpenetrate through the passivation film 31 and the supporting film 32 toextend, respectively, from the anode electrode 25 and the cathodeelectrode 26 to the surface of the supporting film 32. In addition, thefilling electrodes 33 a and 33 b each have an approximately cylindricalshape. The filling electrodes 33 a and 33 b are adapted to connect,respectively, the electrodes 25 and 26 and the bumps 35 a and 35 b to bedescribed hereinafter electrically with each other. The fillingelectrodes 33 a and 33 b are made of Cu, for example. Also, the diameterof the through holes 31 a and 32 a is 10 μm to 200 μm, for example, andpreferably about 100 μm.

On the exposed portions of the filling electrodes 33 a and 33 b at thesurface of the supporting film 32 are formed the UBMs 34 a and 34 b. TheUBMs 34 a and 34 b are composed of accumulation films made of Ni and Au,for example. Also, the thickness of the UBMs 34 a and 34 b is 0.1 μm to5 μm, for example.

On the surfaces of the UBMs 34 a and 34 b on the opposite side of thefilling electrodes 33 a and 33 b are formed the bumps 35 a and 35 b. Thebumps 35 a and 35 b are therefore connected, respectively, to the anodeelectrode 25 and the cathode electrode 26 electrically. The bumps 35 aand 35 b each have an approximately spherical shape except for thesurfaces in contact with the UBMs 34 a and 34 b. As the bumps 35 a and35 b, for example, solder, gold, Ni—Au, Cu, or resin containing metalfiller can be used.

The operation of the back illuminated photodiode 1 will here bedescribed. It is assumed here that the back illuminated photodiode 1 isapplied with a reverse bias voltage, and that there is generated adepletion layer in the thinned area in the N-type semiconductorsubstrate 10. When to-be-detected light penetrates through the coatinglayer 13 and then enters the N-type semiconductor substrate 10 from therecessed portion 12, the light is absorbed mainly in the thinned area.Accordingly, in the area, carriers (holes and electrons) are generated.The generated holes and electrons are moved, respectively, to theP⁺-type doped semiconductor region 11 and the N⁺-type highly-dopedsemiconductor region 22 in accordance with the reverse bias electricfield. Holes and electrons that have reached the P⁺-type dopedsemiconductor region 11 and the N⁺-type highly-doped semiconductorregion 22 are moved to the bumps 35 a and 35 b from the fillingelectrodes 33 a and 33 b and the UBMs 34 a and 34 b to be output as adetected signal from the bumps 35 a and 35 b.

The effect of the back illuminated photodiode 1 will here be described.In the back illuminated photodiode 1, since there is provided thecoating layer 13, the mechanical strength of the back illuminatedphotodiode 1 is increased. In particular, since the coating layer 13 isprovided on the recessed portion 12, it is possible to prevent thethinned area in the N-type semiconductor substrate 10 from beingdistorted, flexed or damaged even if pressure and/or heat may be appliedto the back illuminated photodiode 1 in an assembling operation. Also,the increase in mechanical strength allows for dicing at a wafer level,whereby it is possible to obtain a chip-sized back illuminatedphotodiode 1. Accordingly, there is achieved a back illuminatedphotodiode 1 having a sufficiently small package. In addition, sincethere is no need for a ceramic package, etc., it is possible to reducethe cost of manufacturing a back illuminated photodiode 1. There is thusachieved an inexpensive and highly reliable as well as a small backilluminated photodiode 1.

Further, the coating layer 13 is arranged in such a manner that theportion provided on the recessed portion 12 is sunk lower than theportion provided on the outer edge portion 14 of the recessed portion12. Therefore, even if a flat collet FC may be used in an assemblingoperation as shown in FIG. 2, the surface of the coating layer 13provided on the recessed portion 12 is not brought into contact with theflat collet FC. Thus, the incident part for to-be-detected light withinthe surface of the coating layer 13 cannot be damaged, whereby it ispossible to suppress the scattering of to-be-detected light. Thus, ahighly sensitive back illuminated photodiode 1 is achieved.

In addition, providing the coating layer 13 also on the outer edgeportion 14 prevents the flat collet FC from being brought into directcontact with the outer edge portion 14. It is thus possible to preventthe generation of crystal defects in the outer edge portion 14 due tocontact with the flat collet FC, and therefore to suppress dark currentand/or noise due to the crystal defects.

Also, as the coating layer 13 resin is used, which makes it easy to formthe coating layer 13 into a desired shape.

The provided supporting film 32 further increases the mechanicalstrength of the back illuminated photodiode 1.

The provided filling electrodes 33 a and 33 b make it easy to take adetected signal outside from the electrodes 25 and 26. Additionally, thefilling electrodes 33 a and 33 b may be formed on the sidewalls of thethrough holes 31 a and 32 a to be connected electrically to the anodeelectrode 25 and the cathode electrode 26.

The N⁺-type highly-doped semiconductor layer 21 is formed in the entiresurface layer on the rear surface S2 side of the N-type semiconductorsubstrate 10. The N⁺-type highly-doped semiconductor layer 21 providedin the bottom surface S3 of the recessed portion 12 within the surfacelayer of the rear surface S2 functions as an accumulation layer. Thiscan prevent carriers generated in the N-type semiconductor substrate 10from being recombined in the vicinity of the bottom surface S3. Thus, amore highly sensitive back illuminated photodiode 1 is achieved. Here,the impurity concentration of the N⁺-type highly-doped semiconductorlayer 21 is preferably 10¹⁵/cm³ or more. In this case, the N⁺-typehighly-doped semiconductor layer 21 can suitably function as anaccumulation layer.

Also, even if there may be crystal defects in the outer edge portion 14,the N⁺-type highly-doped semiconductor layer 21, which is provided inthe surface layer on the rear surface S2 side within the outer edgeportion 14 of the N-type semiconductor substrate 10, can suppress darkcurrent and/or noise due to the crystal defects. Therefore, inaccordance with the back illuminated photodiode 1, it is possible toobtain a detected signal at a high S/N ratio. Also, here, the impurityconcentration of the N⁺-type highly-doped semiconductor layer 21 ispreferably 10¹⁵/cm³ or more. In this case, the N⁺-type highly-dopedsemiconductor layer 21 can suppress dark current and/or noise due tocrystal defects sufficiently.

An exemplary method for manufacturing the back illuminated photodiode 1shown in FIG. 1 will here be described with reference to FIG. 3 to FIG.17. First, there is prepared an N-type semiconductor substrate 10 madeof an N-type silicon wafer with the upper surface S1 and the rearsurface S2 thereof being formed into (100) planes. The N-typesemiconductor substrate 10 is thermally oxidized to form an insulatingfilm made of SiO₂ on the upper surface S1 of the N-type semiconductorsubstrate 10. Also, in predetermined portions of the insulating film areformed openings, and then phosphorous is doped into the N-typesemiconductor substrate 10 from the openings to form N⁺-typehighly-doped semiconductor regions 22. Subsequently, the N-typesemiconductor substrate 10 is oxidized to form an insulating film on theupper surface S1. Similarly, in predetermined portions of the insulatingfilm are formed openings, and then boron is doped into the N-typesemiconductor substrate 10 from the openings to form P⁺-type dopedsemiconductor regions 11. Subsequently, the N-type semiconductorsubstrate 10 is oxidized to form an insulating film 23 on the uppersurface S1 (FIG. 3).

Next, the rear surface S2 of the N-type semiconductor substrate 10 ispolished, and SiN 82 is deposited on the rear surface S2 of the N-typesemiconductor substrate 10 by LP-CVD (FIG. 4). Also, in the SiN 82 onthe rear surface S2 are formed openings 85 to form recessed portions 12(FIG. 5). Then, an etching operation is performed using KOH, etc.,through the openings 85 to form recessed portions 12 (FIG. 6).

Next, after the SiN 82 is removed, ion implantation, etc., is performedonto the rear surface S2 side of the N-type semiconductor substrate 10with the recessed portions 12 formed therein to dope N-type impuritiesand thereby to form an N⁺-type highly-doped semiconductor layer 21 inthe entire surface layer on the rear surface S2 side (FIG. 7). Then, thesubstrate is thermally oxidized to form an insulating film 24 in theentire surface layer on the rear surface S2 side (FIG. 8). Contact holesfor electrodes are formed in the insulating film 23 on the upper surfaceS1, and after aluminum is deposited on the upper surface S1, apredetermined pattern is made to form anode electrodes 25 and cathodeelectrodes 26 (FIG. 9).

Next, a passivation film 31 made of SiN is deposited on the uppersurface S1 of the N-type semiconductor substrate 10, on which the anodeelectrodes 25 and the cathode electrodes 26 are formed, by a plasma-CVDmethod. Also, openings 31 a are formed in portions corresponding tobumps 35 a and 35 b within the passivation film 31 (FIG. 10). Further, athick supporting film 32 made of resin is formed on the upper surfaceS1, and openings 32 a are formed in the portions corresponding to theopenings 31 a in the passivation film 31. Here, as the supporting film32, resin such as epoxy-based, acryl-based or polyimide-based one can beused. Alternatively, SiO₂ formed by plasma-CVD, etc., may be used. Also,the openings 32 a in the supporting film 32 can be formed by aphotolithography method using, for example, photosensitive resin or by apatterning process such as etching (FIG. 11). In addition, a conductivematerial 33 made of Cu is deposited in such a manner as to fill theopenings 31 a and 32 a. This can be made through the steps of, forexample, depositing a Cu seed layer, etc., by sputtering, etc., on thesurface of the anode electrodes 25 and the cathode electrodes 26 thatare exposed from the openings 31 a and 32 a, and depositing Cu, etc., byplating on the Cu seed layer (FIG. 12).

Next, the surface of the conductive material 33 is polished to removethe conductive material 33 deposited on the supporting film 32. Thus,filling electrodes 33 a and 33 b are formed (FIG. 13). Also, after acoating layer 13 made of resin is applied by spin coating or printing,etc., in such a manner as to fully cover the rear surface S2, theapplied coating layer 13 is hardened. Here, the portion of the coatinglayer 13 provided on the recessed portion 12 is to be sunk (FIG. 14).Further, UBMs 34 a and 34 b composed of accumulation films made of Niand Au, etc., are formed on the filling electrodes 33 a and 33 b on theupper surface S1 by electroless plating. In addition, bumps 35 a and 35b made of solder, etc., are formed on the UBMs 34 a and 34 b by printingor a ball-mounting method, etc., (FIG. 15).

Finally, in order to obtain individually separated back illuminatedphotodiodes 1, dicing is performed. As indicated by the alternate longand short dashed lines L1 in FIG. 16, the N-type semiconductor substrate10 is diced at the center of each outer edge portion 14 on the rearsurface S2 side. Thus, a back illuminated photodiode 1 (FIG. 17) isobtained.

FIG. 18 is a cross-sectional view showing a second embodiment of a backilluminated photodetector according to the present invention. The backilluminated photodiode 2 comprises a semiconductor substrate 20, aP⁺-type doped semiconductor region 11, a recessed portion 12, and acoating layer 13.

In the surface layer on the upper surface S1 side of the semiconductorsubstrate 20 is partially formed the P⁺-type doped semiconductor region11. Meanwhile, in the rear surface S2 of the semiconductor substrate 20and in an area opposite the P⁺-type doped semiconductor region 11 isformed the recessed portion 12. Also, on the rear surface S2 of thesemiconductor substrate 20 is provided the coating layer 13. The coatinglayer 13 is arranged in such a manner that the portion provided on therecessed portion 12 is sunk lower than the portion provided on the outeredge portion 14 of the recessed portion 12.

The back illuminated photodiode 2 also comprises an N⁺-type highly-dopedsemiconductor region 28, insulating films 23 and 24, an anode electrode25, and a cathode electrode 26. The N⁺-type highly-doped semiconductorregion 28 is formed in such a manner as to be exposed at the entire sidesurfaces S4 of the semiconductor substrate 20. The N⁺-type highly-dopedsemiconductor region 28 also reaches the entire rear surface S2 of thesemiconductor substrate 20. Therefore, the portion 20 a within thesemiconductor substrate 20, in which neither the P⁺-type dopedsemiconductor region 11 nor the N⁺-type highly-doped semiconductorregion 28 is formed, is surrounded entirely by the N⁺-type highly-dopedsemiconductor region 28 from the side surface S4 sides and the rearsurface S2 side of the semiconductor substrate 20.

An exemplary method for forming the N⁺-type highly-doped semiconductorregion 28 will here be described with reference to FIG. 19 to FIG. 21.First, there is prepared a semiconductor substrate 20. In thesemiconductor substrate 20, an N⁺-type highly-doped semiconductor layer41 is diffused from the rear surface S2 with a part on the upper surfaceS1 side remaining. The remaining part on the upper surface S1 side is anN-type doped semiconductor layer 42 having an impurity concentrationlower than that of the N⁺-type highly-doped semiconductor layer 41 (FIG.19). Next, N-type impurities are doped at a high concentration from theupper surface S1 side to form N⁺-type highly-doped semiconductor regions43 (FIG. 20). Then, the N-type impurities are diffused further deeply byheat treatment so that the N⁺-type highly-doped semiconductor regions 43reach the N⁺-type highly-doped semiconductor layer 41 (FIG. 21). Thereis thus formed an N⁺-type highly-doped semiconductor region 28 composedof the N⁺-type highly-doped semiconductor layer 41 and the N⁺-typehighly-doped semiconductor regions 43. Additionally, in FIG. 21, theareas where a P⁺-type doped semiconductor region 11 and a recessedportion 12 are to be formed are indicated, respectively, by the dashedlines L2 and L3. In accordance with the method, since it is possible toomit the step of doping impurities from the rear surface S2 side of thesemiconductor substrate 20, the manufacturing process for the N⁺-typehighly-doped semiconductor region 28 and therefore for the entire backilluminated photodiode 2 is simplified.

Returning to FIG. 18, on the upper surface S1 and the rear surface S2 ofthe semiconductor substrate 20 are formed, respectively, the insulatingfilms 23 and 24. Also, in the insulating film 23 are formed openings 23a and 23 b, one opening 23 a being provided within the range of theP⁺-type doped semiconductor region 11, while the other opening 23 bbeing provided within the range of the N⁺-type highly-dopedsemiconductor region 28.

On the insulating film 23 and in the areas including the openings 23 aand 23 b are formed, respectively, the anode electrode 25 and thecathode electrode 26. The electrodes 25 and 26 are provided in such amanner as to fill the respective openings 23 a and 23 b. Thus, the anodeelectrode 25 is connected directly to the P⁺-type doped semiconductorregion 11 through the opening 23 a, while the cathode electrode 26 isconnected directly to the N⁺-type highly-doped semiconductor region 28through the opening 23 b.

The back illuminated photodiode 2 further comprises a passivation film31, a supporting film 32, filling electrodes 33 a and 33 b, UBMs 34 aand 34 b, and bumps 35 a and 35 b. The passivation film 31 is providedon the upper surface S1 of the semiconductor substrate 20 in such amanner as to cover the insulating film 23, anode electrode 25, andcathode electrode 26. On the passivation film 31 is formed thesupporting film 32. Also, the filling electrodes 33 a and 33 b penetratethrough the passivation film 31 and the supporting film 32 to extend,respectively, from the anode electrode 25 and the cathode electrode 26to the surface of the supporting film 32. On the exposed portions of thefilling electrodes 33 a and 33 b at the surface of the supporting film32 are formed the UBMs 34 a and 34 b. On the surfaces of the UBMs 34 aand 34 b on the opposite side of the filling electrodes 33 a and 33 bare formed the bumps 35 a and 35 b.

The effect of the back illuminated photodiode 2 will here be described.In the back illuminated photodiode 2, since there is provided thecoating layer 13, the mechanical strength of the back illuminatedphotodiode 2 is increased. Also, the increase in mechanical strengthallows for dicing at a wafer level, whereby it is possible to obtain achip-sized back illuminated photodiode 2. Accordingly, there is achieveda back illuminated photodiode 2 having a sufficiently small package.

Further, the coating layer 13 is arranged in such a manner that theportion provided on the recessed portion 12 is sunk lower than theportion provided on the outer edge portion 14 of the recessed portion12. Therefore, even if a flat collet may be used in an assemblingoperation, the surface of the coating layer 13 provided on the recessedportion 12 is not brought into contact with the flat collet. Thus, theincident part for to-be-detected light within the surface of the coatinglayer 13 cannot be damaged, whereby it is possible to suppress thescattering of to-be-detected light. Thus, a highly sensitive backilluminated photodiode 2 is achieved.

Also, in the back illuminated photodiode 2, the N⁺-type highly-dopedsemiconductor region 28 is formed in such a manner as to be exposed atthe entire side surfaces S4 of the semiconductor substrate 20. Thus, theN-type highly-doped semiconductor region 28 can trap unnecessarycarriers that are generated in the vicinity of the side surfaces S4 ofthe semiconductor substrate 20 and thereby can suppress dark currentand/or noise. Although the side surfaces S4 correspond to dicing lineswhereby there is a possibility of causing crystal defects in dicing, theN⁺-type highly-doped semiconductor region 28 can also suppress darkcurrent and/or noise due to the crystal defects. Therefore, inaccordance with the back illuminated photodiode 2, it is possible toobtain a detected signal at a higher S/N ratio.

In addition, the portion 20 a within the semiconductor substrate 20 issurrounded entirely by the N⁺-type highly-doped semiconductor region 28from the side surface S4 sides and the rear surface S2 side of thesemiconductor substrate 20. Thus, a PIN structure in which thesurrounded portion 20 a is employed as an I-layer is achieved. The backilluminated photodiode 2 can be provided with a higher voltage andthereby can increase the width of the depletion layer due to such a PINstructure, which can increase the sensitivity as well as reduce thecapacity thereof to achieve a high-speed response.

FIG. 22 is a plan view showing a third embodiment of a back illuminatedphotodetector according to the present invention. The back illuminatedphotodiode array 3 is composed of a total of sixty-four back illuminatedphotodiodes that are arranged in an eight-by-eight grid pattern. Thearrangement pitch of these photodiodes is 1 mm, for example. FIG. 22shows the appearance of the back illuminated photodiode array 3 whenviewed from the rear surface side. The rear surface of each photodiodeis covered with a coating layer, and is formed in such a manner thatpredetermined portions of the coating layer are sunk, as is the casewith the back illuminated photodiode 1 shown in FIG. 1. In FIG. 22, thesunk portions of the coating layer are indicated by the dashed lines L4.

FIG. 23 is a cross-sectional view of the back illuminated photodiodearray 3 shown in FIG. 22 along the line XII-XII. In the cross-sectionalview are shown two photodiodes P1 and P2 among the sixty-fourphotodiodes shown in FIG. 22. As shown in FIG. 23, the back illuminatedphotodiode array 3 comprises an N-type semiconductor substrate 50, aP⁺-type doped semiconductor region 51, a recessed portion 52, and acoating layer 53.

In the surface layer on the upper surface S1 side of the N-typesemiconductor substrate 50 are formed a plurality of the P⁺-type dopedsemiconductor regions 51. The P⁺-type doped semiconductor regions 51 areprovided, respectively, for the photodiodes P1 and P2. The area of eachP⁺-type doped semiconductor region 51 is 0.75×0.75 mm², for example. Inthe rear surface S2 of the N-type semiconductor substrate 50 and in anarea opposite the P⁺-type doped semiconductor region 51 is formed therecessed portion 52. Here is formed a plurality of the recessed portions52 being provided with a plurality of the P⁺-type doped semiconductorregions 51. In each of the photodiodes P1 and P2 are provided a pair ofa P⁺-type doped semiconductor region 51 and a recessed portion 52. Also,on the rear surface S2 of the N-type semiconductor substrate 50 isprovided the coating layer 53. The coating layer 53 is arranged in sucha manner that the portion provided on the recessed portion 52 is sunklower than the portion provided on the outer edge portion 54 of therecessed portion 52.

The back illuminated photodiode array 3 also comprises an N⁺-typehighly-doped semiconductor layer 61, N⁺-type highly-doped semiconductorregions 62, insulating films 63 and 64, anode electrodes 65, and cathodeelectrodes 66. The N⁺-type highly-doped semiconductor layer 61 is formedin the entire surface layer on the rear surface S2 side of the N-typesemiconductor substrate 50. The N⁺-type highly-doped semiconductorregions 62 are formed in the surface layer on the upper surface S1 sideof the N-type semiconductor substrate 50. The N⁺-type highly-dopedsemiconductor regions 62 are preferably provided in such a manner as tosurround the P⁺-type doped semiconductor regions 51 constituting therespective photodiodes.

On the upper surface S1 and the rear surface S2 of the N-typesemiconductor substrate 50 are formed, respectively, the insulatingfilms 63 and 64. In the insulating film 63 are formed openings 63 a and63 b, some openings 63 a being provided within the range of the P⁺-typedoped semiconductor regions 51, while the other openings 63 b beingprovided within the range of the N⁺-type highly-doped semiconductorregions 62.

On the insulating film 63 and in the areas including the openings 63 aand 63 b are formed, respectively, the anode electrodes 65 and thecathode electrodes 66. In each of the photodiodes P1 and P2 are provideda pair of an anode electrode 65 and a cathode electrode 66. Theelectrodes 65 and 66 are also provided in such a manner as to fill therespective openings 63 a and 63 b. Thus, the anode electrodes 65 areconnected directly to the P⁺-type doped semiconductor regions 51 throughthe respective openings 63 a, while the cathode electrodes 66 areconnected directly to the N⁺-type highly-doped semiconductor regions 62through the respective openings 63 b.

The back illuminated photodiode array 3 further comprises a passivationfilm 71, a supporting film 72, filling electrodes 73 a and 73 b, UBMs 74a and 74 b, and bumps 75 a and 75 b. The passivation film 71 is providedon the upper surface S1 of the N-type semiconductor substrate 50 in sucha manner as to cover the insulating film 63, anode electrodes 65, andcathode electrodes 66. On the passivation film 71 is formed thesupporting film 72. Also, the filling electrodes 73 a and 73 b penetratethrough the passivation film 71 and the supporting film 72 to extend,respectively, from the anode electrodes 65 and the cathode electrodes 66to the surface of the supporting film 72. On the exposed portions of thefilling electrodes 73 a and 73 b at the surface of the supporting film72 are formed the UBMs 74 a and 74 b. On the surfaces of the UBMs 74 aand 74 b on the opposite side of the filling electrodes 73 a and 73 bare formed the bumps 75 a and 75 b.

The effect of the back illuminated photodiode array 3 will here bedescribed. In the back illuminated photodiode array 3, since there isprovided the coating layer 53, the mechanical strength of the backilluminated photodiode array 3 is increased. Also, the increase inmechanical strength allows for dicing at a wafer level, whereby it ispossible to obtain a chip-sized back illuminated photodiode array 3.Accordingly, there is achieved a back illuminated photodiode array 3having a sufficiently small package.

Further, the coating layer 53 is arranged in such a manner that theportions provided on the recessed portions 52 are sunk lower than theportions provided on the outer edge portions 54 of the recessed portions52. Therefore, even if a flat collet may be used in an assemblingoperation, the surface of the coating layer 53 provided on the recessedportions 52 is not brought into contact with the flat collet. Thus, theincident parts for to-be-detected light within the surface of thecoating layer 53 cannot be damaged, whereby it is possible to suppressthe scattering of to-be-detected light. Thus, a highly sensitive backilluminated photodiode array 3 is achieved.

There are also constructed a plurality of photodiodes by forming aplurality of P⁺-type doped semiconductor regions 51 in a plurality ofareas in the surface layer on the upper surface S1 side of the N-typesemiconductor substrate 50, and by forming recessed portions 52 in therear surface S2 and in areas opposite the respective P⁺-type dopedsemiconductor regions 51. Therefore, the back illuminated photodiodearray 3 can suitably be used for an image sensor, etc., in which eachphotodiode represents one pixel.

The back illuminated photodetector according to the present invention isnot restricted to the above-described embodiments, and variousmodifications may be made. For example, in the back illuminatedphotodiode 1 shown in FIG. 1, a P-type semiconductor substrate may beused instead of the N-type semiconductor substrate 10. In this case, thedoped semiconductor region 11 has N-type conductivity, while thehighly-doped semiconductor layer 21 and the highly-doped semiconductorregion 22 have P-type conductivity.

Although in FIG. 12 is shown an example of depositing a conductivematerial 33 made of Cu, Ni may be used instead of Cu to performelectroless plating of Ni directly on the surface of the anodeelectrodes 25 and the cathode electrodes 26 that are exposed from theopenings 31 a and 32 a. In this case, it is possible to omit the step ofpolishing the surface of the conductive material 33 illustrated in FIG.13.

Although in FIG. 15 an example of forming UBMs 34 a and 34 b as well asbumps 35 a and 35 b on the filling electrodes 33 a and 33 b is shown,there is also a method of employing the filling electrodes 33 a and 33 bthemselves as bumps. That is, O₂, etc., is used to dry etch the surfaceof the supporting film 32 with the openings 32 a being filled with thefilling electrodes 33 a and 33 b (refer to FIG. 14). Thus, since thefilling electrodes 33 a and 33 b partially protrude from the surface ofthe supporting film 32, the protruding portions can be used as bumps. Inthis case, it is also not necessary to form UBMs 34 a and 34 b.Alternatively, as a conductive material 33, a conductive resin may beused. This allows the operation of filling the through holes withelectrodes by printing, etc., to be completed in a short time.

Also, in FIG. 19, as the semiconductor substrate 20, a bonded wafer inwhich an N⁺-type highly-doped semiconductor layer and an N-type dopedsemiconductor layer having an impurity concentration lower than that ofthe N⁺-type highly-doped semiconductor layer are bonded to each othermay be used. In this case, the N-type doped semiconductor layer is to beprovided on the upper surface S1 side, while the N⁺-type highly-dopedsemiconductor layer on the rear surface S2 side of the semiconductorsubstrate 20.

INDUSTRIAL APPLICABILITY

In accordance with the present invention, it is possible to achieve aback illuminated back illuminated having a sufficiently small package aswell as being capable of suppressing the scattering of to-be-detectedlight.

1. A back illuminated photodetector comprising: a first conductor typesemiconductor substrate; a second conductive type doped semiconductorregion provided in the first superficial surface layer of thesemiconductor substrate; a recessed portion for incidence ofto-be-detected light formed in the second surface of the semiconductorsubstrate and in an area opposite the doped semiconductor region; and acoating layer made of resin for transmitting the to-be-detected light,the coating layer being provided on the second surface, the coatinglayer being arranged in such a manner that the portion provided on therecessed portion in the second surface is sunk lower than the portionprovided on the outer edge portion of the recessed portion.
 2. The backilluminated photodetector according to claim 1, further comprising asupporting film provided on the first surface of the semiconductorsubstrate to support the semiconductor substrate.
 3. The backilluminated photodetector according to claim 2, further comprising afilling electrode penetrating through the supporting film and connectedelectrically to the doped semiconductor region at the one end thereof.4. The back illuminated photodetector according to claim 1, wherein ahighly-doped semiconductor region with impurities of the firstconductive type added thereto at a high concentration is exposed acrossthe entire side surface of the semiconductor substrate.
 5. The backilluminated photodetector according to claim 1, wherein a highly-dopedsemiconductor layer with impurities of the first conductive type addedthereto at a high concentration is provided in the bottom portion of therecessed portion within the second superficial surface layer of thesemiconductor substrate.
 6. The back illuminated photodector accordingto claim 1, wherein a highly-doped semiconductor layer with impuritiesof the first conductive type added thereto at a high concentration isprovided in the second superficial surface layer in the outer edgeportion of the semiconductor substrate.